Recognition and response in the plant immune system

Heterologous expression of wheat TaRUB1 gene enhances disease resistance in Arabidopsis thaliana

Expression of TaRUB1 gene in Arabidopsis thaliana elevates the level of disease-related genes in response to pathogen invasion through the accumulation of callose, necrotic cells, and the outbreak of ROS. Ubiquitin (Ub) and ubiquitin-like proteins are highly conserved in sequence and can covalently bind and modify many intracellular proteins which can be recognized and degraded by 26S proteasome. Post-translational modification of proteins has become a hot research spot today. In the previous study, a cDNA of related-to-ubiquitin protein belonged to ubiquitin-like proteins, whose spatial structure comprised Ub and NEDD8, was obtained from wheat SN6306 by suppression-subtractive hybridization and was named TaRUB1. TaRUB1 is induced by wheat powdery mildew and significantly upregulated in resistant wheat SN6306. In this study, heterologous expression of TaRUB1 in A. thaliana was used to study the function of this gene in response to pathogen Pseudomonas syringae pv. Tomato DC3000 (Pst DC3000). Transgenic A. thaliana showed relatively fewer disease symptoms, accompanied by common inhibition of living body parasitic defense responses, accumulation of more callose and reactive oxygen species (ROS), and concentrated cell death, simultaneously antioxidant enzyme activities of superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase were higher than those in wild-type (WT) plant after infection with Pst DC3000. Meanwhile, hypersensitive cell death, which was possibly ROS burst, was also observed in transgenic A. thaliana. By quantitative reverse transcription-polymerase chain reaction analysis, some marker genes for hypersensitive response showed significantly higher transcriptional expression level in transgenic A. thaliana, which activates system-acquired resistance, than that of WT plants. Heterologous expression of TaRUB1 can significantly enhance resistance to Pst DC3000 in A. thaliana, suggesting that TaRUB1 is related to plant disease resistance.

Differential Interaction of Synthetic Glycolipids with Biomimetic Plasma Membrane Lipids Correlates with the Plant Biological Response

Natural and synthetic amphiphilic molecules including lipopeptides, lipopolysaccharides, and glycolipids are able to induce defense mechanisms in plants. In the present work, the perception of two synthetic C14 rhamnolipids, namely, Alk-RL and Ac-RL, differing only at the level of the lipid tail terminal group have been investigated using biological and biophysical approaches. We showed that Alk-RL induces a stronger early signaling response in tobacco cell suspensions than does Ac-RL. The interactions of both synthetic RLs with simplified biomimetic membranes were further analyzed using experimental and in silico approaches. Our results indicate that the interactions of Alk-RL and Ac-RL with lipids were different in terms of insertion and molecular responses and were dependent on the lipid composition of model membranes. A more favorable insertion of Alk-RL than Ac-RL into lipid membranes is observed. Alk-RL forms more stable molecular assemblies than Ac-RL with phospholipids and sterols. At the molecular level, the presence of sterols tends to increase the RLs' interaction with lipid bilayers, with a fluidizing effect on the alkyl chains. Taken together, our findings suggest that the perception of these synthetic RLs at the membrane level could be related to a lipid-driven process depending on the organization of the membrane and the orientation of the RLs within the membrane and is correlated with the induction of early signaling responses in tobacco cells.

JMJ27, an Arabidopsis H3K9 histone demethylase, modulates defense against Pseudomonas syringae and flowering time

Histone methylation is known to dynamically regulate diverse developmental and physiological processes. Histone methyl marks are written by methyltransferases and erased by demethylases, and result in modification of chromatin structure to repress or activate transcription. However, little is known about how histone methylation may regulate defense mechanisms and flowering time in plants. Here we report characterization of JmjC DOMAIN-CONTAINING PROTEIN 27 (JMJ27), an Arabidopsis JHDM2 (JmjC domain-containing histone demethylase 2) family protein, which modulates defense against pathogens and flowering time. JMJ27 is a nuclear protein containing a zinc-finger motif and a catalytic JmjC domain with conserved Fe(II) and α-ketoglutarate binding sites, and displays H3K9me1/2 demethylase activity both in vitro and in vivo. JMJ27 is induced in response to virulent Pseudomonas syringae pathogens and is required for resistance against these pathogens. JMJ27 is a negative modulator of WRKY25 (a repressor of defense) and a positive modulator of several pathogenesis-related (PR) proteins. Additionally, loss of JMJ27 function leads to early flowering. JMJ27 negatively modulates the major flowering regulator CONSTANS (CO) and positively modulates FLOWERING LOCUS C (FLC). Taken together, our results indicate that JMJ27 functions as a histone demethylase to modulate both physiological (defense) and developmental (flowering time) processes in Arabidopsis.

CBL-interacting protein kinase 6 negatively regulates immune response to Pseudomonas syringae in Arabidopsis

Cytosolic calcium ion (Ca2+) is an essential mediator of the plant innate immune response. Here, we report that a calcium-regulated protein kinase Calcineurin B-like protein (CBL)-interacting protein kinase 6 (CIPK6) functions as a negative regulator of immunity against the bacterial pathogen Pseudomonas syringae in Arabidopsis thaliana. Arabidopsis lines with compromised expression of CIPK6 exhibited enhanced disease resistance to the bacterial pathogen and to P. syringae harboring certain but not all avirulent effectors, while restoration of CIPK6 expression resulted in abolition of resistance. Plants overexpressing CIPK6 were more susceptible to P. syringae. Enhanced resistance in the absence of CIPK6 was accompanied by increased accumulation of salicylic acid and elevated expression of defense marker genes. Salicylic acid accumulation was essential for improved immunity in the absence of CIPK6. CIPK6 negatively regulated the oxidative burst associated with perception of pathogen-associated microbial patterns (PAMPs) and bacterial effectors. Accelerated and enhanced activation of the mitogen-activated protein kinase cascade in response to bacterial and fungal elicitors was observed in the absence of CIPK6. The results of this study suggested that CIPK6 negatively regulates effector-triggered and PAMP-triggered immunity in Arabidopsis.

The Arabidopsis Elongator complex is required for nonhost resistance against the bacterial pathogens Xanthomonas citri subsp. citri and Pseudomonas syringae pv. phaseolicola NPS3121

Although in recent years nonhost resistance has attracted considerable attention for its broad spectrum and durability, the genetic and mechanistic components of nonhost resistance have not been fully understood. We used molecular and histochemical approaches including quantitative PCR, chromatin immunoprecipitation, and 3,3'-diaminobenzidine and aniline blue staining. The evolutionarily conserved histone acetyltransferase complex Elongator was identified as a major component of nonhost resistance against Xanthomonas citri subsp. citri (Xcc) and Pseudomonas syringae pv. phaseolicola (Psp) NPS3121. Mutations in Elongator genes inhibit Xcc-, Psp NPS3121- and/or flg22-induced defense responses including defense gene expression, callose deposition, and reactive oxygen species (ROS) and salicylic acid (SA) accumulation. Mutations in Elongator also attenuate the ROS-SA amplification loop. We show that suppressed ROS and SA accumulation in Elongator mutants is correlated with reduced expression of the Arabidopsis respiratory burst oxidase homologue AtrbohD and the SA biosynthesis gene ISOCHORISMATE SYNTHASE1 (ICS1). Furthermore, we found that the Elongator subunit ELP2 is associated with the chromatin of AtrbohD and ICS1 and is required for maintaining basal histone H3 acetylation levels in these key defense genes. As both AtrbohD and ICS1 contribute to nonhost resistance against Xcc, our results reveal an epigenetic mechanism by which Elongator regulates nonhost resistance in Arabidopsis.

Arabidopsis thaliana methionine sulfoxide reductase B8 influences stress-induced cell death and effector-triggered immunity

Reactive oxygen species (ROS) oxidize methionine to methionine sulfoxide (MetSO) and thereby inactivate proteins. Methionine sulfoxide reductase (MSR) enzyme converts MetSO back to the reduced form and thereby detoxifies the effect of ROS. Our results show that Arabidopsis thaliana MSR enzyme coding gene MSRB8 is required for effector-triggered immunity and containment of stress-induced cell death in Arabidopsis. Plants activate pattern-triggered immunity (PTI), a basal defense, upon recognition of evolutionary conserved molecular patterns present in the pathogens. Pathogens release effector molecules to suppress PTI. Recognition of certain effector molecules activates a strong defense, known as effector-triggered immunity (ETI). ETI induces high-level accumulation of reactive oxygen species (ROS) and hypersensitive response (HR), a rapid programmed death of infected cells. ROS oxidize methionine to methionine sulfoxide (MetSO), rendering several proteins nonfunctional. The methionine sulfoxide reductase (MSR) enzyme converts MetSO back to the reduced form and thereby detoxifies the effect of ROS. Though a few plant MSR genes are known to provide tolerance against oxidative stress, their role in plant-pathogen interaction is not known. We report here that activation of cell death by avirulent pathogen or UV treatment induces expression of MSRB7 and MSRB8 genes. The T-DNA insertion mutant of MSRB8 exaggerates HR-associated and UV-induced cell death and accumulates a higher level of ROS than wild-type plants. The negative regulatory role of MSRB8 in HR is further supported by amiRNA and overexpression lines. Mutants and overexpression lines of MSRB8 are susceptible and resistant respectively, compared to the wild-type plants, against avirulent strains of Pseudomonas syringae pv. tomato DC3000 (Pst) carrying AvrRpt2, AvrB, or AvrPphB genes. However, the MSRB8 gene does not influence resistance against virulent Pst or P. syringae pv. maculicola (Psm) pathogens. Our results altogether suggest that MSRB8 function is required for ETI and containment of stress-induced cell death in Arabidopsis.

Cellulase from Trichoderma harzianum interacts with roots and triggers induced systemic resistance to foliar disease in maize

Trichoderma harzianum is well known to exhibit induced systemic resistance (ISR) to Curvularia leaf spot. We previously reported that a C6 zinc finger protein (Thc6) is responsible for a major contribution to the ISR to the leaf disease, but the types of effectors and the signals mediated by Thc6 from Trichoderma are unclear. In this work, we demonstrated that two hydrolases, Thph1 and Thph2, from T. harzianum were regulated by Thc6. Furthermore, an electrophoretic mobility shift assay (EMSA) study revealed that Thc6 regulated mRNA expression by binding to GGCTAA and GGCTAAA in the promoters of the Thph1 and Thph2 genes, respectively. Moreover, the Thph1 and Thph2 proteins triggered the transient production of reactive oxygen species (ROS) and elevated the free cytosolic calcium levels in maize leaf. Furthermore, the genes related to the jasmonate/ethylene signaling pathway were up-regulated in the wild-type maize strain. However, the ΔThph1- or ΔThph2-deletion mutants could not activate the immune defense-related genes in maize to protect against leaf disease. Therefore, we conclude that functional Thph1 and Thph2 may be required in T. harzianum to activate ISR in maize.

Members of WRKY Group III transcription factors are important in TYLCV defense signaling pathway in tomato (Solanum lycopersicum)

Background

Transmitted by the whitefly Bemisia tabaci, tomato yellow leaf curly virus (TYLCV) has posed serious threats to plant growth and development. Plant innate immune systems against various threats involve WRKY Group III transcription factors (TFs). This group participates as a major component of biological processes in plants.

Results

In this study, 6 WRKY Group III TFs (SolyWRKY41, SolyWRKY42, SolyWRKY53, SolyWRKY54, SolyWRKY80, and SolyWRKY81) were identified, and these TFs responded to TYLCV infection. Subcellular localization analysis indicated that SolyWRKY41 and SolyWRKY54 were nuclear proteins in vivo. Many elements, including W-box, were found in the promoter region of Group III TFs. Interaction network analysis revealed that Group III TFs could interact with other proteins, such as mitogen-activated protein kinase 5 (MAPK) and isochorismate synthase (ICS), to respond to biotic and abiotic stresses. Positive and negative expression patterns showed that WRKY Group III genes could also respond to TYLCV infection in tomato. The DNA content of TYLCV resistant lines after SolyWRKY41 and SolyWRKY54 were subjected to virus-induced gene silencing (VIGS) was lower than that of the control lines.

Conclusions

In the present study, 6 WRKY Group III TFs in tomato were identified to respond to TYLCV infection. Quantitative real-time–polymerase chain reaction (RT-qPCR) and VIGS analyses demonstrated that Group III genes served as positive and negative regulators in tomato–TYLCV interaction. WRKY Group III TFs could interact with other proteins by binding to cis elements existing in the promoter regions of other genes to regulate pathogen-related gene expression.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3123-2) contains supplementary material, which is available to authorized users.

Analysis of Sequenced Genomes of Xanthomonas perforans Identifies Candidate Targets for Resistance Breeding in Tomato

Bacterial disease management is a challenge for modern agriculture due to rapid changes in pathogen populations. Genome sequences for hosts and pathogens provide detailed information that facilitates effector-based breeding strategies. Tomato genotypes have gene-for-gene resistance to the bacterial spot pathogen Xanthomonas perforans. The bacterial spot populations in Florida shifted from tomato race 3 to 4, such that the corresponding tomato resistance gene no longer recognizes the effector protein AvrXv3. Genome sequencing showed variation in effector profiles among race 4 strains collected in 2006 and 2012 and compared with a race 3 strain collected in 1991. We examined variation in putative targets of resistance among Florida strains of X. perforans collected from 1991 to 2006. Consistent with race change, avrXv3 was present in race 3 strains but nonfunctional in race 4 strains due to multiple independent mutations. Effectors xopJ4 and avrBs2 were unchanged in all strains. The effector avrBsT was absent in race 3 strains collected in the 1990s but present in race 3 strains collected in 2006 and nearly all race 4 strains. These changes in effector profiles suggest that xopJ4 and avrBsT are currently the best targets for resistance breeding against bacterial spot in tomato.

The Synthetic Elicitor DPMP (2,4-dichloro-6-{(E)-[(3-methoxyphenyl)imino]methyl}phenol) Triggers Strong Immunity in Arabidopsis thaliana and Tomato

Synthetic elicitors are drug-like compounds that are structurally distinct from natural defense elicitors. They can protect plants from diseases by activating host immune responses and can serve as tools for the dissection of the plant immune system as well as leads for the development of environmentally-safe pesticide alternatives. By high-throughput screening, we previously identified 114 synthetic elicitors that activate expression of the pathogen-responsive CaBP22⁻³³³::GUS reporter gene in Arabidopsis thaliana (Arabidopsis), 33 of which are [(phenylimino)methyl]phenol (PMP) derivatives or PMP-related compounds. Here we report on the characterization of one of these compounds, 2,4-dichloro-6-{(E)-[(3-methoxyphenyl)imino]methyl}phenol (DPMP). DPMP strongly triggers disease resistance of Arabidopsis against bacterial and oomycete pathogens. By mRNA-seq analysis we found transcriptional profiles triggered by DPMP to resemble typical defense-related responses.

Innate Immunity to Intracellular Pathogens: Lessons Learned from Legionella pneumophila

Intracellular bacterial pathogens have the remarkable ability to manipulate host cell processes in order to establish a replicative niche within the host cell. In response, the host can initiate immune defenses that lead to the eventual restriction and clearance of intracellular infection. The bacterial pathogen Legionella pneumophila has evolved elaborate virulence mechanisms that allow for its survival inside protozoa within a specialized membrane-bound organelle. These strategies also enable L. pneumophila to survive and replicate within alveolar macrophages, and can result in the severe pneumonia Legionnaires' disease. Essential to L. pneumophila's intracellular lifestyle is a specialized type IV secretion system, termed Dot/Icm, that translocates bacterial effector proteins into host cells. The ease with which L. pneumophila can be genetically manipulated has facilitated the comparison of host responses to virulent and isogenic avirulent mutants lacking a functional Dot/Icm system. This has made L. pneumophila an excellent model for understanding how the host discriminates between pathogenic and nonpathogenic bacteria and for systematically dissecting host defense mechanisms against intracellular pathogens. In this chapter, I discuss a few examples demonstrating how the study of immune responses triggered specifically by the L. pneumophila type IV secretion system has provided unique insight into our understanding of host immunity against intracellular bacterial pathogens.

The syntaxin protein (MoSyn8) mediates intracellular trafficking to regulate conidiogenesis and pathogenicity of rice blast fungus

Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) mediate cellular membrane fusion and intracellular vesicle trafficking in eukaryotic cells, and are critical in the growth and development of pathogenic fungi such as Magnaporthe oryzae which causes rice blast. Rice blast is thought to involve distinct SNARE-mediated transport and secretion of fungal effector proteins into the host to modulate rice immunity. We have previously characterized two SNARE proteins, secretory protein (MoSec22) and vesicle-associated membrane protein (MoVam7), as being important in cellular transport and pathogenicity. Here, we show that syntaxin 8 (MoSyn8), a Qc-SNARE protein homolog, also plays important roles in growth, conidiation, and pathogenicity. The MoSYN8 deletion mutant (∆Mosyn8) mutant exhibits defects in endocytosis and F-actin organization, appressorium turgor pressure generation, and host penetration. In addition, the ∆Mosyn8 mutant cannot elaborate biotrophic invasion of the susceptible rice host, or secrete avirulence factors Avr-Pia (corresponding to the rice resistance gene Pia) and Avrpiz-t (the cognate Avr gene for the resistance gene Piz-t) proteins. Our study of MoSyn8 advances our understanding of SNARE proteins in effector secretion which underlies the normal physiology and pathogenicity of M. oryzae, and it sheds new light on the mechanism of the blight disease caused by M. oryzae.

Proteomic Analyses Provide Novel Insights into Plant Growth and Ginsenoside Biosynthesis in Forest Cultivated Panax ginseng (F. Ginseng)

F. Ginseng (Panax ginseng) is planted in the forest to enhance the natural ginseng resources, which have an immense medicinal and economic value. The morphology of the cultivated plants becomes similar to that of wild growing ginseng (W. Ginseng) over the years. So far, there have been no studies highlighting the physiological or functional changes in F. Ginseng and its wild counterparts. In the present study, we used proteomic technologies (2DE and iTRAQ) coupled to mass spectrometry to compare W. Ginseng and F. Ginseng at various growth stages. Hierarchical cluster analysis based on protein abundance revealed that the protein expression profile of 25-year-old F. Ginseng was more like W. Ginseng than less 20-year-old F. Ginseng. We identified 192 differentially expressed protein spots in F. Ginseng. These protein spots increased with increase in growth years of F. Ginseng and were associated with proteins involved in energy metabolism, ginsenosides biosynthesis, and stress response. The mRNA, physiological, and metabolic analysis showed that the external morphology, protein expression profile, and ginsenoside synthesis ability of the F. Ginseng increased just like that of W. Ginseng with the increase in age. Our study represents the first characterization of the proteome of F. Ginseng during development and provides new insights into the metabolism and accumulation of ginsenosides.

Arabidopsis thaliana: A Model Host Plant to Study Plant-Pathogen Interaction Using Rice False Smut Isolates of Ustilaginoidea virens

Rice false smut fungus which is a biotrophic fungal pathogen causes an important rice disease and brings a severe damage where rice is cultivated. We established a new fungal-plant pathosystem where Ustilaginoidea virens was able to interact compatibly with the model plant Arabidopsis thaliana. Disease symptoms were apparent on the leaves of the plants after 6 days of post inoculation in the form of chlorosis. Cytological studies showed that U. virens caused a heavy infestation inside the cells of the chlorotic tissues. Development and colonization of aerial mycelia in association with floral organ, particularly on anther and stigma of the flowers after 3 weeks of post inoculation was evident which finally caused infection on the developing seeds and pod tissues. The fungus adopts a uniquely biotrophic infection strategy in roots and spreads without causing a loss of host cell viability. We have also demonstrated that U. virens isolates infect Arabidopsis and the plant subsequently activates different defense response mechanisms which are witnessed by the expression of pathogenesis-related genes, PR-1, PR-2, PR-5, PDF1.1, and PDF1.2. The established A. thaliana-U. virens pathosystem will now permit various follow-up molecular genetics and gene expression experiments to be performed to identify the defense signals and responses that restrict fungal hyphae colonization in planta and also provide initial evidence for tissue-adapted fungal infection strategies.

The different interactions of Colletotrichum gloeosporioides with two strawberry varieties and the involvement of salicylic acid

The disease symptoms recognized as 'Anthracnose' are caused by Colletotrichum spp. and lead to large-scale strawberry (Fragaria×ananassa Duchesne) losses worldwide in terms of both quality and production. Little is known regarding the mechanisms underlying the genetic variations in the strawberry-Colletotrichum spp. interaction. In this work, Colletotrichum gloeosporioides (C. gloeosporioides) infection was characterized in two varieties exhibiting different susceptibilities, and the involvement of salicylic acid (SA) was examined. Light microscopic observation showed that C. gloeosporioides conidia germinated earlier and faster on the leaf surface of the susceptible cultivar compared with the less-susceptible cultivar. Several PR genes were differentially expressed, with higher-amplitude changes observed in the less-susceptible cultivar. The less-susceptible cultivar contained a higher level of basal SA, and the SA levels increased rapidly upon infection, followed by a sharp decrease before the necrotrophic phase. External SA pretreatment reduced susceptibility and elevated the internal SA levels in both varieties, which were sharply reduced in the susceptible cultivar upon inoculation. The less-susceptible cultivar also displayed a more sensitive and marked increase in the transcripts of NB-LRR genes to C. gloeosporioides, and SA pretreatment differentially induced transcript accumulation in the two varieties during infection. Furthermore, SA directly inhibited the germination of C. gloeosporioides conidia; NB-LRR transcript accumulation in response to SA pretreatment was both dose- and cultivar-dependent. The results demonstrate that the less-susceptible cultivar showed reduced conidia germination. The contribution of SA might involve microbial isolate-specific sensitivity to SA, cultivar/tissue-specific SA homeostasis and signaling, and the sensitivity of R genes and the related defense network to SA and pathogens.

The Cerato-Platanin protein Epl-1 from Trichoderma harzianum is involved in mycoparasitism, plant resistance induction and self cell wall protection

Trichoderma harzianum species are well known as biocontrol agents against important fungal phytopathogens. Mycoparasitism is one of the strategies used by this fungus in the biocontrol process. In this work, we analyzed the effect of Epl-1 protein, previously described as plant resistance elicitor, in expression modulation of T. harzianum genes involved in mycoparasitism process against phytopathogenic fungi; self cell wall protection and recognition; host hyphae coiling and triggering expression of defense-related genes in beans plants. The results indicated that the absence of Epl-1 protein affects the expression of all mycoparasitism genes analyzed in direct confrontation assays against phytopathogen Sclerotinia sclerotiorum as well as T. harzianum itself; the host mycoparasitic coiling process and expression modulation of plant defense genes showing different pattern compared with wild type strain. These data indicated the involvement T. harzianum Epl-1 in self and host interaction and also recognition of T. harzianum as a symbiotic fungus by the bean plants.

Rice WRKY4 acts as a transcriptional activator mediating defense responses toward Rhizoctonia solani, the causing agent of rice sheath blight

WRKY transcription factors have been implicated in the regulation of transcriptional reprogramming associated with various plant processes but most notably with plant defense responses to pathogens. Here we demonstrate that expression of rice WRKY4 gene (OsWRKY4) was rapidly and strongly induced upon infection of Rhizoctonia solani, the causing agent of rice sheath blight, and exogenous jasmonic acid (JA) and ethylene (ET). OsWRKY4 is localized to the nucleus of plant cells and possesses transcriptional activation ability. Modulation of OsWRKY4 transcript levels by constitutive overexpression increases resistance to the necrotrophic sheath blight fungus, concomitant with elevated expression of JA- and ET-responsive pathogenesis-related (PR) genes such as PR1a, PR1b, PR5 and PR10/PBZ1. Suppression by RNA interference (RNAi), on the other hand, compromises resistance to the fungal pathogen. Yeast one-hybrid assay and transient expression in tobacco cells reveal that OsWRKY4 specifically binds to the promoter regions of PR1b and PR5 which contain W-box (TTGAC[C/T]), or W-box like (TGAC[C/T]) cis-elements. In conclusion, we propose that OsWRKY4 functions as an important positive regulator that is implicated in the defense responses to rice sheath blight via JA/ET-dependent signal pathway.

Repression of microRNA biogenesis by silencing of OsDCL1 activates the basal resistance to Magnaporthe oryzae in rice

The RNaseIII enzyme Dicer-like 1 (DCL1) processes the microRNA biogenesis and plays a determinant role in plant development. In this study, we reported the function of OsDCL1 in the immunity to rice blast, the devastating disease caused by the fungal pathogen, Magnaporthe oryzae. Expression profiling demonstrated that different OsDCLs responded dynamically and OsDCL1 reduced its expression upon the challenge of rice blast pathogen. In contrast, miR162a predicted to target OsDCL1 increased its expression, implying a negative feedback loop between OsDCL1 and miR162a in rice. In addition to developmental defects, the OsDCL1-silencing mutants showed enhanced resistance to virulent rice blast strains in a non-race specific manner. Accumulation of hydrogen peroxide and cell death were observed in the contact cells with infectious hyphae, revealing that silencing of OsDCL1 activated cellular defense responses. In OsDCL1 RNAi lines, 12 differentially expressed miRNAs were identified, of which 5 and 7 were down- and up-regulated, respectively, indicating that miRNAs responded dynamically in the interaction between rice and rice blast. Moreover, silencing of OsDCL1 activated the constitutive expression of defense related genes. Taken together, our results indicate that rice is capable of activating basal resistance against rice blast by perturbing OsDCL1-dependent miRNA biogenesis pathway.

Aboveground insect infestation attenuates belowground Agrobacterium-mediated genetic transformation

Agrobacterium tumefaciens causes crown gall disease. Although Agrobacterium can be popularly used for genetic engineering, the influence of aboveground insect infestation on Agrobacterium induced gall formation has not been investigated. Nicotiana benthamiana leaves were exposed to a sucking insect (whitefly) infestation and benzothiadiazole (BTH) for 7 d, and these exposed plants were inoculated with a tumorigenic Agrobacterium strain. We evaluated, both in planta and in vitro, how whitefly infestation affects crown gall disease. Whitefly-infested plants exhibited at least a two-fold reduction in gall formation on both stem and crown root. Silencing of isochorismate synthase 1 (ICS1), required for salicylic acid (SA) synthesis, compromised gall formation indicating an involvement of SA in whitefly-derived plant defence against Agrobacterium. Endogenous SA content was augmented in whitefly-infested plants upon Agrobacterium inoculation. In addition, SA concentration was three times higher in root exudates from whitefly-infested plants. As a consequence, Agrobacterium-mediated transformation of roots of whitefly-infested plants was clearly inhibited when compared to control plants. These results suggest that aboveground whitefly infestation elicits systemic defence responses throughout the plant. Our findings provide new insights into insect-mediated leaf-root intra-communication and a framework to understand interactions between three organisms: whitefly, N. benthamiana and Agrobacterium.

Enhancement of innate immune system in monocot rice by transferring the dicotyledonous elongation factor Tu receptor EFR

The elongation factor Tu (EF-Tu) receptor (EFR) in cruciferous plants specifically recognizes the N-terminal acetylated elf18 region of bacterial EF-Tu and thereby activates plant immunity. It has been demonstrated that Arabidopsis EFR confers broad-spectrum bacterial resistance in the EFR transgenic solanaceous plants. Here, the transgenic rice plants (Oryza sativa L. ssp. japonica cv. Zhonghua 17) and cell cultures with constitutive expression of AtEFR were developed to investigate whether AtEFR senses EF-Tu and thus enhances bacterial resistance in the monocot plants. We demonstrated that the Xanthomonas oryzae-derived elf18 peptide induced oxidative burst and mitogen-activated protein kinase activation in the AtEFR transgenic rice cells and plants, respectively. Pathogenesis-related genes, such as OsPBZ1, were upregulated dramatically in transgenic rice plant and cell lines in response to elf18 stimulation. Importantly, pretreatment with elf18 triggered strong resistance to X. oryzae pv. oryzae in the transgenic plants, which was largely dependent on the AtEFR expression level. These plants also exhibited enhanced resistance to rice bacterial brown stripe, but not to rice fungal blast. Collectively, the results indicate that the rice plants with heterologous expression of AtEFR recognize bacterial EF-Tu and exhibit enhanced broad-spectrum bacterial disease resistance and that pattern recognition receptor-mediated immunity may be manipulated across the two plant classes, dicots and monocots.

Host versus nonhost resistance: distinct wars with similar arsenals

Plants face several challenges by bacterial, fungal, oomycete, and viral pathogens during their life cycle. In order to defend against these biotic stresses, plants possess a dynamic, innate, natural immune system that efficiently detects potential pathogens and initiates a resistance response in the form of basal resistance and/or resistance (R)-gene-mediated defense, which is often associated with a hypersensitive response. Depending upon the nature of plant-pathogen interactions, plants generally have two main defense mechanisms, host resistance and nonhost resistance. Host resistance is generally controlled by single R genes and less durable compared with nonhost resistance. In contrast, nonhost resistance is believed to be a multi-gene trait and more durable. In this review, we describe the mechanisms of host and nonhost resistance against fungal and bacterial plant pathogens. In addition, we also attempt to compare host and nonhost resistance responses to identify similarities and differences, and their practical applications in crop improvement.

Apoplastic peroxidases are required for salicylic acid-mediated defense against Pseudomonas syringae

Reactive oxygen species (ROS) generated by NADPH oxidases or apoplastic peroxidases play an important role in the plant defense response. Diminished expression of at least two Arabidopsis thaliana peroxidase encoding genes, PRX33 (At3g49110) and PRX34 (At3g49120), as a consequence of anti-sense expression of a heterologous French bean peroxidase gene (asFBP1.1), were previously shown to result in reduced levels of ROS following pathogen attack, enhanced susceptibility to a variety of bacterial and fungal pathogens, and reduced levels of callose production and defense-related gene expression in response to the microbe associated molecular pattern (MAMP) molecules flg22 and elf26. These data demonstrated that the peroxidase-dependent oxidative burst plays an important role in the elicitation of pattern-triggered immunity (PTI). Further work reported in this paper, however, shows that asFBP1.1 antisense plants are not impaired in all PTI-associated responses. For example, some but not all flg22-elicited genes are induced to lower levels by flg22 in asFPB1.1, and callose deposition in asFPB1.1 is similar to wild-type following infiltration with a Pseudomonas syringae hrcC mutant or with non-host P. syringae pathovars. Moreover, asFPB1.1 plants did not exhibit any apparent defect in their ability to mount a hypersensitive response (HR). On the other hand, salicylic acid (SA)-mediated activation of PR1 was dramatically impaired in asFPB1.1 plants. In addition, P. syringae-elicited expression of many genes known to be SA-dependent was significantly reduced in asFBP1.1 plants. Consistent with this latter result, in asFBP1.1 plants the key regulator of SA-mediated responses, NPR1, showed both dramatically decreased total protein abundance and a failure to monomerize, which is required for its translocation into the nucleus.

Synthetic plant defense elicitors

To defend themselves against invading pathogens plants utilize a complex regulatory network that coordinates extensive transcriptional and metabolic reprogramming. Although many of the key players of this immunity-associated network are known, the details of its topology and dynamics are still poorly understood. As an alternative to forward and reverse genetic studies, chemical genetics-related approaches based on bioactive small molecules have gained substantial popularity in the analysis of biological pathways and networks. Use of such molecular probes can allow researchers to access biological space that was previously inaccessible to genetic analyses due to gene redundancy or lethality of mutations. Synthetic elicitors are small drug-like molecules that induce plant defense responses, but are distinct from known natural elicitors of plant immunity. While the discovery of some synthetic elicitors had already been reported in the 1970s, recent breakthroughs in combinatorial chemical synthesis now allow for inexpensive high-throughput screens for bioactive plant defense-inducing compounds. Along with powerful reverse genetics tools and resources available for model plants and crop systems, comprehensive collections of new synthetic elicitors will likely allow plant scientists to study the intricacies of plant defense signaling pathways and networks in an unparalleled fashion. As synthetic elicitors can protect crops from diseases, without the need to be directly toxic for pathogenic organisms, they may also serve as promising alternatives to conventional biocidal pesticides, which often are harmful for the environment, farmers and consumers. Here we are discussing various types of synthetic elicitors that have been used for studies on the plant immune system, their modes-of-action as well as their application in crop protection.

Optimal level of purple acid phosphatase5 is required for maintaining complete resistance to Pseudomonas syringae

Plants possess an exceedingly complex innate immune system to defend against most pathogens. However, a relative proportion of the pathogens overcome host's innate immunity and impair plant growth and productivity. We previously showed that mutation in purple acid phosphatase (PAP5) lead to enhanced susceptibility of Arabidopsis to the bacterial pathogen Pseudomonas syringae pv. tomato DC3000 (Pst DC3000). Here, we report that an optimal level of PAP5 is crucial for mounting complete basal resistance. Overexpression of PAP5 impaired ICS1, PR1 expression and salicylic acid (SA) accumulation similar to pap5 knockout mutant plants. Moreover, plant overexpressing PAP5 was impaired in H2O2 accumulation in response to Pst DC3000. PAP5 is localized in to peroxisomes, a known site of generation of reactive oxygen species for activation of defense responses. Taken together, our results demonstrate that optimal levels of PAP5 is required for mounting resistance against Pst DC3000 as both knockout and overexpression of PAP5 lead to compromised basal resistance.

Isolation and characterization of a Solanum tuberosum subtilisin-like protein with caspase-3 activity (StSBTc-3)

Plant proteases with caspase-like enzymatic activity have been widely studied during the last decade. Previously, we have reported the presence and induction of caspase-3 like activity in the apoplast of potato leaves during Solanum tuberosum- Phytophthora infestans interaction. In this work we have purified and identified a potato extracellular protease with caspase-3 like enzymatic activity from potato leaves infected with P. infestans. Results obtained from the size exclusion chromatography show that the isolated protease is a monomeric enzyme with an estimated molecular weight of 70 kDa approximately. Purified protease was analyzed by MALDI-TOF MS, showing a 100% of sequence identity with the deduced amino acid sequence of a putative subtilisin-like protease from S. tuberosum (Solgenomics protein ID: PGSC0003DMP400018521). For this reason the isolated protease was named as StSBTc-3. This report constitutes the first evidence of isolation and identification of a plant subtilisin-like protease with caspase-3 like enzymatic activity. In order to elucidate the possible function of StSBTc-3 during plant pathogen interaction, we demonstrate that like animal caspase-3, StSBTc-3 is able to produce in vitro cytoplasm shrinkage in plant cells and to induce plant cell death. This result suggest that, StSBTc-3 could exert a caspase executer function during potato- P. infestans interaction, resulting in the restriction of the pathogen spread during plant-pathogen interaction.

Interconnection between flowering time control and activation of systemic acquired resistance

The ability to avoid or neutralize pathogens is inherent to all higher organisms including plants. Plants recognize pathogens through receptors, and mount resistance against the intruders, with the help of well-elaborated defense arsenal. In response to some localinfections, plants develop systemic acquired resistance (SAR), which provides heightened resistance during subsequent infections. Infected tissues generate mobile signaling molecules that travel to the systemic tissues, where they epigenetically modify expression o a set of genes to initiate the manifestation of SAR in distant tissues. Immune responses are largely regulated at transcriptional level. Flowering is a developmental transition that occurs as a result of the coordinated action of large numbers of transcription factors that respond to intrinsic signals and environmental conditions. The plant hormone salicylic acid (SA) which is required for SAR activation positively regulates flowering. Certain components of chromatin remodeling complexes that are recruited for suppression of precocious flowering are also involved in suppression of SAR in healthy plants. FLOWERING LOCUS D, a putative histone demethylase positively regulates SAR manifestation and flowering transition in Arabidopsis. Similarly, incorporation of histone variant H2A.Z in nucleosomes mediated by PHOTOPERIOD-INDEPENDENT EARLY FLOWERING 1, an ortholog of yeast chromatin remodeling complex SWR1, concomitantly influences SAR and flowering time. SUMO conjugation and deconjugation mechanisms also similarly affect SAR and flowering in an SA-dependent manner. The evidences suggest a common underlying regulatory mechanism for activation of SAR and flowering in plants.

Temperature-induced viral resistance in Emiliania huxleyi (Prymnesiophyceae)

Annual Emiliania huxleyi blooms (along with other coccolithophorid species) play important roles in the global carbon and sulfur cycles. E. huxleyi blooms are routinely terminated by large, host-specific dsDNA viruses, (Emiliania huxleyi Viruses; EhVs), making these host-virus interactions a driving force behind their potential impact on global biogeochemical cycles. Given projected increases in sea surface temperature due to climate change, it is imperative to understand the effects of temperature on E. huxleyi's susceptibility to viral infection and its production of climatically active dimethylated sulfur species (DSS). Here we demonstrate that a 3°C increase in temperature induces EhV-resistant phenotypes in three E. huxleyi strains and that successful virus infection impacts DSS pool sizes. We also examined cellular polar lipids, given their documented roles in regulating host-virus interactions in this system, and propose that alterations to membrane-bound surface receptors are responsible for the observed temperature-induced resistance. Our findings have potential implications for global biogeochemical cycles in a warming climate and for deciphering the particular mechanism(s) by which some E. huxleyi strains exhibit viral resistance.

The molecular ecophysiology of programmed cell death in marine phytoplankton

Planktonic, prokaryotic, and eukaryotic photoautotrophs (phytoplankton) share a diverse and ancient evolutionary history, during which time they have played key roles in regulating marine food webs, biogeochemical cycles, and Earth's climate. Because phytoplankton represent the basis of marine ecosystems, the manner in which they die critically determines the flow and fate of photosynthetically fixed organic matter (and associated elements), ultimately constraining upper-ocean biogeochemistry. Programmed cell death (PCD) and associated pathway genes, which are triggered by a variety of nutrient stressors and are employed by parasitic viruses, play an integral role in determining the cell fate of diverse photoautotrophs in the modern ocean. Indeed, these multifaceted death pathways continue to shape the success and evolutionary trajectory of diverse phytoplankton lineages at sea. Research over the past two decades has employed physiological, biochemical, and genetic techniques to provide a novel, comprehensive, mechanistic understanding of the factors controlling this key process. Here, I discuss the current understanding of the genetics, activation, and regulation of PCD pathways in marine model systems; how PCD evolved in unicellular photoautotrophs; how it mechanistically interfaces with viral infection pathways; how stress signals are sensed and transduced into cellular responses; and how novel molecular and biochemical tools are revealing the impact of PCD genes on the fate of natural phytoplankton assemblages.

Calmodulin Gene Expression in Response to Mechanical Wounding and Botrytis cinerea Infection in Tomato Fruit

Calmodulin, a ubiquitous calcium sensor, plays an important role in decoding stress-triggered intracellular calcium changes and regulates the functions of numerous target proteins involved in various plant physiological responses. To determine the functions of calmodulin in fleshy fruit, expression studies were performed on a family of six calmodulin genes (SlCaMs) in mature-green stage tomato fruit in response to mechanical injury and Botrytis cinerea infection. Both wounding and pathogen inoculation triggered expression of all those genes, with SlCaM2 being the most responsive one to both treatments. Furthermore, all calmodulin genes were upregulated by salicylic acid and methyl jasmonate, two signaling molecules involved in plant immunity. In addition to SlCaM2, SlCaM1 was highly responsive to salicylic acid and methyl jasmonate. However, SlCaM2 exhibited a more rapid and stronger response than SlCaM1. Overexpression of SlCaM2 in tomato fruit enhanced resistance to Botrytis-induced decay, whereas reducing its expression resulted in increased lesion development. These results indicate that calmodulin is a positive regulator of plant defense in fruit by activating defense pathways including salicylate- and jasmonate-signaling pathways, and SlCaM2 is the major calmodulin gene responsible for this event.

Rice SAPs are responsive to multiple biotic stresses and overexpression of OsSAP1, an A20/AN1 zinc-finger protein, enhances the basal resistance against pathogen infection in tobacco

Eukaryotic A20/AN1 zinc-finger proteins (ZFPs) play an important role in the regulation of immune and stress response. After elucidation of the role of first such protein, OsSAP1, in abiotic stress tolerance, 18 rice stress associated protein (SAP) genes have been shown to be regulated by multiple abiotic stresses. In the present study, expression pattern of all the 18 OsSAP genes have been analysed in response to different biotic stress simulators, in order to get insights into their possible involvement in biotic stress tolerance. Our results showed the upregulation of OsSAP1 and OsSAP11 by all biotic stress simulator treatments. Furthermore, the functional role of OsSAP1 in plant defence responses has been explored through overexpression in transgenic plants. Constitutive expression of OsSAP1 in transgenic tobacco resulted into enhanced disease resistance against virulent bacterial pathogen, together with the upregulation of known defence-related genes. Present investigation suggests that rice SAPs are responsive to multiple biotic stresses and OsSAP1 plays a key role in basal resistance against pathogen infection. This strongly supports the involvement of rice SAPs in cross-talk between biotic and abiotic stress signalling pathways, which makes them ideal candidate to design strategies for protecting crop plants against multiple stresses.

High-throughput screening of small-molecule libraries for inducers of plant defense responses

Transgenic Arabidopsis seedlings containing a pathogen-responsive reporter gene allow for convenient high-throughput screening of chemical libraries for compounds that induce plant defense responses. Candidates identified by such screens can be further tested for their ability to protect plants from pathogen-caused diseases. Using Arabidopsis defense signaling mutants, defined regulatory processes that are targeted by a given candidate molecule can be easily narrowed down. Here, we provide a detailed high-throughput screening protocol for library compounds that activate a pathogen-responsive reporter gene in liquid-grown Arabidopsis seedlings.

The bacterial lipopeptide iturins induce Verticillium dahliae cell death by affecting fungal signalling pathways and mediate plant defence responses involved in pathogen-associated molecular pattern-triggered immunity

Verticillium wilt in cotton caused by Verticillium dahliae is one of the most serious plant diseases worldwide. Because no known fungicides or cotton cultivars provide sufficient protection against this pathogen, V. dahliae causes major crop yield losses. Here, an isolated cotton endophytic bacterium, designated Bacillus amyloliquefaciens 41B-1, exhibited greater than 50% biocontrol efficacy against V. dahliae in cotton plants under greenhouse conditions. Through high-performance liquid chromatography and mass analysis of the filtrate, we found that the antifungal compounds present in the strain 41B-1 culture filtrate were a series of isoforms of iturins. The purified iturins suppressed V. dahliae microsclerotial germination in the absence or presence of cotton. Treatment with the iturins induced reactive oxygen species bursts, Hog1 mitogen-activated protein kinase (MAPK) activation and defects in cell wall integrity. The oxidative stress response and high-osmolarity glycerol pathway contribute to iturins resistance in V. dahliae. In contrast, the Slt2 MAPK pathway may be involved in iturins sensitivity in this fungus. In addition to antagonism, iturins could induce plant defence responses as activators and mediate pathogen-associated molecular pattern-triggered immunity. These findings suggest that iturins may affect fungal signalling pathways and mediate plant defence responses against V. dahliae.

Treatment of Amaranthus cruentus with chemical and biological inducers of resistance has contrasting effects on fitness and protection against compatible Gram positive and Gram negative bacterial pathogens

Amaranthus cruentus (Ac) plants were treated with the synthetic systemic acquired resistance (SAR) inducer benzothiadiazole (BTH), methyl jasmonate (MeJA) and the incompatible pathogen, Pseudomonas syringae pv. syringae (Pss), under greenhouse conditions. The treatments induced a set of marker genes in the absence of pathogen infection: BTH and Pss similarly induced genes coding for pathogenesis-related and antioxidant proteins, whereas MeJA induced the arginase, LOX2 and amarandin 1 genes. BTH and Pss were effective when tested against the Gram negative pathogen Ps pv. tabaci (Pst), which was found to have a compatible interaction with grain amaranth. The resistance response appeared to be salicylic acid-independent. However, resistance against Clavibacter michiganensis subsp. michiganensis (Cmm), a Gram positive tomato pathogen also found to infect Ac, was only conferred by Pss, while BTH increased susceptibility. Conversely, MeJA was ineffective against both pathogens. Induced resistance against Pst correlated with the rapid and sustained stimulation of the above genes, including the AhPAL2 gene, which were expressed both locally and distally. The lack of protection against Cmm provided by BTH, coincided with a generalized down-regulation of defense gene expression and chitinase activity. On the other hand, Pss-treated Ac plants showed augmented expression levels of an anti-microbial peptide gene and, surprisingly, of AhACCO, an ethylene biosynthetic gene associated with susceptibility to Cmm in tomato, its main host. Pss treatment had no effect on productivity, but compromised growth, whereas MeJA reduced yield and harvest index. Conversely, BTH treatments led to smaller plants, but produced significantly increased yields. These results suggest essential differences in the mechanisms employed by biological and chemical agents to induce SAR in Ac against bacterial pathogens having different infection strategies. This may determine the outcome of a particular plant-pathogen interaction, leading to resistance or susceptibility, as in Cmm-challenged Ac plants previously induced with Pss or BTH, respectively.

Preparing to fight back: generation and storage of priming compounds

Immune-stimulated plants are able to respond more rapidly and adequately to various biotic stresses allowing them to efficiently combat an infection. During the priming phase, plant are stimulated in absence of a challenge, and can accumulate and store conjugates or precursors of molecules as well as other compounds that play a role in defense. These molecules can be released during the defensive phase following stress. These metabolites can also participate in the first stages of the stress perception. Here, we report the metabolic changes occuring in primed plants during the priming phase. β-aminobutyric acid (BABA) causes a boost of the primary metabolism through the tricarboxylic acids (TCA) such as citrate, fumarate, (S)-malate and 2-oxoglutarate, and the potentiation of phenylpropanoid biosynthesis and the octodecanoic pathway. On the contrary, Pseudomonas syringae pv tomato (PstAvrRpt2) represses the same pathways. Both systems used to prime plants share some common signals like the changes in the synthesis of amino acids and the production of SA and its glycosides, as well as IAA. Interestingly, a product of the purine catabolism, xanthosine, was found to accumulate following both BABA- and PstAvrRpt2-treatement. The compounds that are strongly affected in this stage are called priming compounds, since their effect on the metabolism of the plant is to induce the production of primed compounds that will help to combat the stress. At the same time, additional identified metabolites suggest the possible defense pathways that plants are using to get ready for the battle.

Structure-based computational study of two disease resistance gene homologues (Hm1 and Hm2) in maize (Zea mays L.) with implications in plant-pathogen interactions

The NADPH-dependent HC-toxin reductases (HCTR1 and 2) encoded by enzymatic class of disease resistance homologous genes (Hm1 and Hm2) protect maize by detoxifying a cyclic tetrapeptide, HC-toxin, secreted by the fungus Cochliobolus carbonum race 1(CCR1). Unlike the other classes' resistance (R) genes, HCTR-mediated disease resistance is an inimitable mechanism where the avirulence (Avr) component from CCR1 is not involved in toxin degradation. In this study, we attempted to decipher cofactor (NADPH) recognition and mode of HC-toxin binding to HCTRs through molecular docking, molecular dynamics (MD) simulations and binding free energy calculation methods. The rationality and the stability of docked complexes were validated by 30-ns MD simulation. The binding free energy decomposition of enzyme-cofactor complex was calculated to find the driving force behind cofactor recognition. The overall binding free energies of HCTR1-NADPH and HCTR2-NADPH were found to be −616.989 and −16.9749 kJ mol⁻¹ respectively. The binding free energy decomposition revealed that the binding of NADPH to the HCTR1 is mainly governed by van der Waals and nonpolar interactions, whereas electrostatic terms play dominant role in stabilizing the binding mode between HCTR2 and NADPH. Further, docking analysis of HC-toxin with HCTR-NADPH complexes showed a distinct mode of binding and the complexes were stabilized by a strong network of hydrogen bond and hydrophobic interactions. This study is the first in silico attempt to unravel the biophysical and biochemical basis of cofactor recognition in enzymatic class of R genes in cereal crop maize.

Identification and characterization of the grape WRKY family

WRKY transcription factors have functions in plant growth and development and in response to biotic and abiotic stresses. Many studies have focused on functional identification of WRKY transcription factors, but little is known about the molecular phylogeny or global expression patterns of the complete WRKY family. In this study, we identified 80 WRKY proteins encoded in the grape genome. Based on the structural features of these proteins, the grape WRKY genes were classified into three groups (groups 1–3). Analysis of WRKY genes expression profiles indicated that 28 WRKY genes were differentially expressed in response to biotic stress caused by grape whiterot and/or salicylic acid (SA). In that 16 WRKY genes upregulated both by whiterot pathogenic bacteria and SA. The results indicated that 16 WRKY proteins participated in SA-dependent defense signal pathway. This study provides a basis for cloning genes with specific functions from grape.

Regulation of plant immunity through ubiquitin-mediated modulation of Ca(2+) -calmodulin-AtSR1/CAMTA3 signaling

Transient changes in intracellular Ca(2+) concentration are essential signals for activation of plant immunity. It has also been reported that Ca(2+) signals suppress salicylic acid-mediated plant defense through AtSR1/CAMTA3, a member of the Ca(2+) /calmodulin-regulated transcription factor family that is conserved in multicellular eukaryotes. How plants overcome this negative regulation to mount an effective defense response during a stage of intracellular Ca(2+) surge is unclear. Here we report the identification and functional characterization of an important component of ubiquitin ligase, and the associated AtSR1 turnover. The AtSR1 interaction protein 1 (SR1IP1) was identified by CytoTrap two-hybrid screening. The loss-of-function mutant of SR1IP1 is more susceptible to bacterial pathogens, and over-expression of SR1IP1 confers enhanced resistance, indicating that SR1IP1 acts as a positive regulator of plant defense. SR1IP1 and AtSR1 act in the same signaling pathway to regulate plant immunity. SR1IP1 contains the structural features of a substrate adaptor in cullin 3-based E3 ubiquitin ligase, and was shown to serve as a substrate adaptor that recruits AtSR1 for ubiquitination and degradation when plants are challenged with pathogens. Hence, SR1IP1 positively regulates plant immunity by removing the defense suppressor AtSR1. These findings provide a mechanistic insight into how Ca(2+) -mediated actions are coordinated to achieve effective plant immunity.

Functions of Calcium-Dependent Protein Kinases in Plant Innate Immunity

An increase of cytosolic Ca²⁺ is generated by diverse physiological stimuli and stresses, including pathogen attack. Plants have evolved two branches of the immune system to defend against pathogen infections. The primary innate immune response is triggered by the detection of evolutionarily conserved pathogen-associated molecular pattern (PAMP), which is called PAMP-triggered immunity (PTI). The second branch of plant innate immunity is triggered by the recognition of specific pathogen effector proteins and known as effector-triggered immunity (ETI). Calcium (Ca²⁺) signaling is essential in both plant PTI and ETI responses. Calcium-dependent protein kinases (CDPKs) have emerged as important Ca²⁺ sensor proteins in transducing differential Ca²⁺ signatures, triggered by PAMPs or effectors and activating complex downstream responses. CDPKs directly transmit calcium signals by calcium binding to the elongation factor (EF)-hand domain at the C-terminus and substrate phosphorylation by the catalytic kinase domain at the N-terminus. Emerging evidence suggests that specific and overlapping CDPKs phosphorylate distinct substrates in PTI and ETI to regulate diverse plant immune responses, including production of reactive oxygen species, transcriptional reprogramming of immune genes, and the hypersensitive response.

Overexpression of CaWRKY27, a subgroup IIe WRKY transcription factor of Capsicum annuum, positively regulates tobacco resistance to Ralstonia solanacearum infection

WRKY proteins are encoded by a large gene family and are linked to many biological processes across a range of plant species. The functions and underlying mechanisms of WRKY proteins have been investigated primarily in model plants such as Arabidopsis and rice. The roles of these transcription factors in non-model plants, including pepper and other Solanaceae, are poorly understood. Here, we characterize the expression and function of a subgroup IIe WRKY protein from pepper (Capsicum annuum), denoted as CaWRKY27. The protein localized to nuclei and activated the transcription of a reporter GUS gene construct driven by the 35S promoter that contained two copies of the W-box in its proximal upstream region. Inoculation of pepper cultivars with Ralstonia solanacearum induced the expression of CaWRKY27 transcript in 76a, a bacterial wilt-resistant pepper cultivar, whereas it downregulated the expression of CaWRKY27 transcript in Gui-1-3, a bacterial wilt-susceptible pepper cultivar. CaWRKY27 transcript levels were also increased by treatments with salicylic acid (SA), methyl jasmonate (MeJA) and ethephon (ETH). Transgenic tobacco plants overexpressing CaWRKY27 exhibited resistance to R. solanacearum infection compared to that of wild-type plants. This resistance was coupled with increased transcript levels in a number of marker genes, including hypersensitive response genes, and SA-, JA- and ET-associated genes. By contrast, virus-induced gene silencing (VIGS) of CaWRKY27 increased the susceptibility of pepper plants to R. solanacearum infection. These results suggest that CaWRKY27 acts as a positive regulator in tobacco resistance responses to R. solanacearum infection through modulation of SA-, JA- and ET-mediated signaling pathways.

The Arabidopsis oligopeptidases TOP1 and TOP2 are salicylic acid targets that modulate SA-mediated signaling and the immune response

Salicylic acid (SA) is a small phenolic molecule with hormonal properties, and is an essential component of the immune response. SA exerts its functions by interacting with protein targets; however, the specific cellular components modulated by SA and critical for immune signal transduction are largely unknown. To uncover cellular activities targeted by SA, we probed Arabidopsis protein microarrays with a functional analog of SA. We demonstrate that thimet oligopeptidases (TOPs) constitute a class of SA-binding enzymes. Biochemical evidence demonstrated that SA interacts with TOPs and inhibits their peptidase activities to various degrees both in vitro and in plant extracts. Functional characterization of mutants with altered TOP expression indicated that TOP1 and TOP2 mediate SA-dependent signaling and are necessary for the immune response to avirulent pathogens. Our results support a model whereby TOP1 and TOP2 act in separate pathways to modulate SA-mediated cellular processes.

Endoplasmic reticulum glucosidases and protein quality control factors cooperate to establish biotrophy in Ustilago maydis

Secreted fungal effectors mediate plant-fungus pathogenic interactions. These proteins are typically N-glycosylated, a common posttranslational modification affecting their location and function. N-glycosylation consists of the addition, and subsequent maturation, of an oligosaccharide core in the endoplasmic reticulum (ER) and Golgi apparatus. In this article, we show that two enzymes catalyzing specific stages of this pathway in maize smut (Ustilago maydis), glucosidase I (Gls1) and glucosidase II β-subunit (Gas2), are essential for its pathogenic interaction with maize (Zea mays). Gls1 is required for the initial stages of infection following appressorium penetration, and Gas2 is required for efficient fungal spreading inside infected tissues. While U. maydis Δgls1 cells induce strong plant defense responses, Δgas2 hyphae are able to repress them, showing that slight differences in the N-glycoprotein processing can determine the extent of plant-fungus interactions. Interestingly, the calnexin protein, a central element of the ER quality control system for N-glycoproteins in eukaryotic cells, is essential for avoiding plant defense responses in cells with defective N-glycoproteins processing. Thus, N-glycoprotein maturation and this conserved checkpoint appear to play an important role in the establishment of an initial biotrophic state with the plant, which allows subsequent colonization.

An RxLR effector from Phytophthora infestans prevents re-localisation of two plant NAC transcription factors from the endoplasmic reticulum to the nucleus

The potato late blight pathogen Phytophthora infestans secretes an array of effector proteins thought to act in its hosts by disarming defences and promoting pathogen colonisation. However, little is known about the host targets of these effectors and how they are manipulated by the pathogen. This work describes the identification of two putative membrane-associated NAC transcription factors (TF) as the host targets of the RxLR effector PITG_03192 (Pi03192). The effector interacts with NAC Targeted by Phytophthora (NTP) 1 and NTP2 at the endoplasmic reticulum (ER) membrane, where these proteins are localised. Transcripts of NTP1 and NTP2 rapidly accumulate following treatment with culture filtrate (CF) from in vitro grown P. infestans, which acts as a mixture of Phytophthora PAMPs and elicitors, but significantly decrease during P. infestans infection, indicating that pathogen activity may prevent their up-regulation. Silencing of NTP1 or NTP2 in the model host plant Nicotiana benthamiana increases susceptibility to P. infestans, whereas silencing of Pi03192 in P. infestans reduces pathogenicity. Transient expression of Pi03192 in planta restores pathogenicity of the Pi03192-silenced line. Moreover, colonisation by the Pi03192-silenced line is significantly enhanced on N. benthamiana plants in which either NTP1 or NTP2 have been silenced. StNTP1 and StNTP2 proteins are released from the ER membrane following treatment with P. infestans CF and accumulate in the nucleus, after which they are rapidly turned over by the 26S proteasome. In contrast, treatment with the defined PAMP flg22 fails to up-regulate NTP1 and NTP2, or promote re-localisation of their protein products to the nucleus, indicating that these events follow perception of a component of CF that appears to be independent of the FLS2/flg22 pathway. Importantly, Pi03192 prevents CF-triggered re-localisation of StNTP1 and StNTP2 from the ER into the nucleus, revealing a novel effector mode-of-action to promote disease progression.

Recent advances in calcium/calmodulin-mediated signaling with an emphasis on plant-microbe interactions

Calcium/calmodulin-mediated signaling contributes in diverse roles in plant growth, development, and response to environmental stimuli .

Insights into the structure-function relationship of disease resistance protein HCTR in maize (Zea mays L.): a computational structural biology approach

The disease resistance gene Hm1 of maize encodes a NADPH-dependent reductase enzyme, HC-toxin reductase (HCTR) that detoxifies the HC toxin secreted by the race specific fungus Cochliobolus carbonum race 1. HCTR enzyme shares 29.6% sequence identity with dihydroflavonol reductase (DFR) of grape, a key enzyme involved in flavonoid biosynthesis. Here we report the comparative modelling, molecular dynamics simulation and docking studies to explain the structure-function relationship and the mode of cofactor (NADPH) binding in HCTR enzyme at the molecular level. The nucleotide binding domain of modelled HCTR adopts a classic Rossmann fold and possesses a consensus glycine rich GxGxxG motif. Molecular simulation studies suggested that HCTR model retained stability throughout the simulation in aqueous solution. HCTR model showed considerable structural identities with the cofactor binding site of DFR, but significant difference in the catalytic site might be the reason of functional divergence between these families of proteins. Similarly electrostatic surface potential analysis of both HCTR and DFR revealed profound variations in the charge distribution over the substrate binding site, which can be correlated with the sequence variability and may suggest distinct substrate-binding patterns and differences in the catalytic mechanism. Docking results indicated Phe19, Gly21, Arg40, Thr90, Gly208, Arg218, Glu221 and Thr222 are important residues for cofactor (NADPH) binding through strong hydrogen bonding and electrostatic interactions. Alanine scanning and analysis of docking energies of mutant proteins suggested that Phe19, and Arg40 are two critical residues for the cofactor binding. The result from the present study is expected to pave the way for exploration of similar genes in other economically important crop varieties.

Evaluation of higher plant virus resistance genes in the green alga, Chlorella variabilis NC64A, during the early phase of infection with Paramecium bursaria chlorella virus-1

With growing industrial interest in algae plus their critical roles in aquatic systems, the need to understand the effects of algal pathogens is increasing. We examined a model algal host-virus system, Chlorella variabilis NC64A and virus, PBCV-1. C. variabilis encodes 375 homologs to genes involved in RNA silencing and in response to virus infection in higher plants. Illumina RNA-Seq data showed that 325 of these homologs were expressed in healthy and early PBCV-1 infected (≤60min) cells. For each of the RNA silencing genes to which homologs were found, mRNA transcripts were detected in healthy and infected cells. C. variabilis, like higher plants, may employ certain RNA silencing pathways to defend itself against virus infection. To our knowledge this is the first examination of RNA silencing genes in algae beyond core proteins, and the first analysis of their transcription during virus infection.

Characterization of the LOV1-mediated, victorin-induced, cell-death response with virus-induced gene silencing

Victoria blight, caused by Cochliobolus victoriae, is a disease originally described on oat and recapitulated on Arabidopsis. C. victoriae pathogenesis depends upon production of the toxin victorin. In oat, victorin sensitivity is conferred by the Vb gene, which is genetically inseparable from the Pc2 resistance gene. Concurrently, in Arabidopsis, sensitivity is conferred by the LOCUS ORCHESTRATING VICTORIN EFFECTS1 (LOV1) gene. LOV1 encodes a nucleotide-binding site leucine-rich repeat protein, a type of protein commonly associated with disease resistance, and LOV1 "guards" the defense thioredoxin, TRX-h5. Expression of LOV1 and TRX-h5 in Nicotiana benthamiana is sufficient to confer victorin sensitivity. Virus-induced gene silencing was used to characterize victorin-induced cell death in N. benthamiana. We determined that SGT1 is required for sensitivity and involved in LOV1 protein accumulation. We screened a normalized cDNA library and identified six genes that, when silenced, suppressed LOV1-mediated, victorin-induced cell death and cell death induced by expression of the closely related RPP8 resistance gene: a mitochondrial phosphate transporter, glycolate oxidase, glutamine synthetase, glyceraldehyde 3-phosphate dehydrogenase, and the P- and T-protein of the glycine decarboxylase complex. Silencing the latter four also inhibited cell death and disease resistance mediated by the PTO resistance gene. Together, these results provide evidence that the victorin response mediated by LOV1 is a defense response.

Effect of external and internal factors on the expression of reporter genes driven by the N resistance gene promoter

The role of resistance (R) genes in plant pathogen interaction has been studied extensively due to its economical impact on agriculture. Interaction between tobacco mosaic virus (TMV) and the N protein from tobacco is one of the most widely used models to understand various aspects of pathogen resistance. The transcription activity governed by N gene promoter is one of the least understood elements of the model. In this study, the N gene promoter was cloned and fused with two different reporter genes, one encoding β-glucuronidase (N::GUS) and another, luciferase (N::LUC). Tobacco plants transformed with the N::GUS or N::LUC reporter constructs were screened for homozygosity and stable expression. Histochemical analysis of N::GUS tobacco plants revealed that the expression is organ specific and developmentally regulated. Whereas two week old plants expressed GUS in midveins only, 6-wk-old plants also expressed GUS in leaf lamella. Roots did not show GUS expression at any time during development. Experiments to address effects of external stress were performed using N::LUC tobacco plants. These experiments showed that N gene promoter expression was suppressed when plants were exposed to high but not low temperatures. Expression was also upregulated in response to TMV, but no changes were observed in plants treated with SA.